iPhone 5 screen stops responding

Hi

Strange problem that my iPhone screen intermittently stops responding and to get it working again I need to lock the phone then unlock again. In addition to this I sometime have white squares across the top of the screen.

I have tried numerous resets and I am sure this is a software problem as the screen is fine again every time after lock/unlocking. My current iOS version is 9.0.2 (13A452)

My iPhone was replaced earlier this year at an apple store and it cost me £200. I am worried that if it is broken they will charge this again for another replacement. The cost of ownership is becoming unreasonable.

Please can anyone advise on what I should do next?

Many thanks in advance.